The present invention relates to a nonaqueous electrolyte secondary cell, and more particularly to a carbon material for a negative electrode of a lithium ion secondary cell.
A nonaqueous electrolyte secondary cell, or the so-called lithium secondary cell, has been previously studied in the interest of obtaining higher energy density, by achieving a higher voltage and a larger capacity. This secondary cell uses lithium metal as an active material in the negative electrode, and an oxide, sulfide, selenide or other chalcogen compound of transition metals, such as manganese dioxide, molybdenum disulfide or titanium selenide, as an active material in the positive electrode. The nonaqueous electrolyte is an organic electrolyte, made of an organic solvent solution of lithium salt. In this lithium secondary cell, however, while an interlayer compound which exhibits relatively good charge and discharge characteristics may be selected as a positive electrode active material, the charge and discharge characteristics of metal lithium for a negative electrode are not particularly favorable.
Thus, the cycle life for repeated charge and discharge can hardly be extended. Moreover, there is a danger that an internal short circuit may generate heat, presenting a safety problem. More specifically, the lithium metal in the negative electrode can elute into the organic electrolyte as lithium ions. electrode as metal lithium by charge, not all of them precipitate smoothly as in the initial state, but some precipitate as active metal crystals in the form of dendrite or moss. The active metal crystals decompose the organic solvent in the electrolyte, while the surface of the metal crystals is covered with a passive film to be inactivated, hardly contributing to discharge. As a result, as charge and discharge cycles are repeated, the negative electrode capacity declines, wherefore the negative electrode capacity had to be set extremely larger than that of the positive electrode when fabricating a cell. Besides, the active dendritic metal lithium crystals may pierce through the separator and contact with the positive electrode, possibly causing internal short-circuit. By internal short-circuit, the cell may generate heat.
Accordingly, the so-called lithium secondary cell, which uses a carbon material as the negative electrode material, has been proposed. This latter cell is capable of reversibly undergoing repeated intercalation and deintercalation with each charge and discharge, and it is now being intensively researched and developed, and it is already in actual use. In this lithium secondary cell, so far as it is not overcharged, active dendritic metal lithium crystals do not precipitate on the negative electrode surface when the cell is charging up and discharging, and enhancement of safety is much expected. Moreover, since this battery is extremely superior in high rate charge and discharge characteristics and cycle life to the lithium secondary cell using metal lithium in the negative electrode active material, the demand for this battery is growing rapidly in recent years.
As the positive electrode active material for lithium ion secondary cell of 4V class, a composite oxide of lithium and transition metal, such as LiCoO2, LiNiO2, LiMnO2, and LiMn2O4, corresponding to the discharge state is being employed or considered. As the electrolyte, similarly as in the lithium secondary cell, a nonaqueous electrolyte such as organic electrolyte and polymer solid electrolyte is used.
When graphite is used in the negative electrode material, the theoretical value of capacity per 1 g of carbon by reference to C6Li of interlayer compound produced by intercalation of lithium ion is 372 mAh. Therefore, among various carbon materials, the one which helps realize a specific capacity close to this theoretical value, as well as causes the capacity per unit volume, i.e., capacity density (mAh/cc) to be as high as possible, should be selected for the negative electrode that is put in practical use.
Among various carbon materials, in the hardly graphitized carbon generally known as hard carbon, materials which exhibit a specific capacity exceeding the above mentioned theoretical value (372 mAh/g) are discovered and are being investigated. However, since the hardly graphitized amorphous carbon is small in true specific gravity and is bulky, it is substantially difficult to increase the capacity density per unit volume of the negative electrode. Furthermore, there still remain many problems, for example, the negative electrode potential after charge is not so base as to be close to the metal lithium potential, and flatness of discharge potential is inferior.
By contrast, when natural graphite or artificial graphite powder which is high in crystallinity is used in the negative electrode, the potential after charge is close to the metal lithium potential, and the flatness of discharge potential is excellent, whereby the charge and discharge characteristics are enhanced as a practical battery, and thus the graphite powder is recently becoming the mainstream of negative electrode material.
However, when the mean particle size of the graphite powder for negative electrode of a lithium ion secondary cell is large, the charge and discharge characteristics at high rate and discharge characteristic at low temperature tend to be inferior. Accordingly, by reducing the mean particle size of the powder, the high rate charge and discharge characteristics and low temperature discharge characteristic are enhanced, but if the mean particle size is made too small, the specific surface area of the powder becomes too large, as a result of which there is a problem of increased irreversible capacity, in which the lithium inserted by first charge in the powder cannot contribute to discharge after the first cycle. This phenomenon is not only a fatal demerit for enhancement of energy density, but also causes the solvent in the organic electrolyte to be decomposed in case the battery is left at a high temperature exceeding 100xc2x0 C., which may lead to self-discharge as well as an electrolyte leak accident due to raise in the cell internal pressure, thereby lowering the reliability of the battery.
It is hence easily understood that the appropriate specific surface area and mean particle size are essential for the graphite powder for negative electrode. An invention proposed from such viewpoint is, for example, Japanese Laid-open Patent No. 6-295725, which uses graphite powder of which specific surface area by BET method is 1 to 10 m2/g, mean particle size is 10 to 30 microns, and at least one of the content of powder with a particle size of 10 microns or less and the content of powder with a particle size of 30 microns or more is 10% or less. Further, in Japanese Laid-open Patent No. 7-134988, the usage of spherical graphite powder is disclosed, which is obtained by graphitizing meso-carbon micro-beads formed by heating petroleum pitch at a low temperature and of which plane interval (d002) of (002) plane by wide angle X-ray diffraction method is 3.36 to 3.40 angstroms, and specific surface area by BET method is 0.7 to 5.0 m2/g. Further, Japanese Laid-open Patent No. 5-307959 discloses the use of a multi-phase carbon matter having a specific surface area that is 20 m2/g or less as well as less than half of the specific surface area of a nucleus of carbon matter.
These inventions were not only extremely effective for enhancement of high rate charge and discharge characteristics and discharge characteristic at low temperature of the lithium ion secondary cell, but also effective for decreasing the irreversible capacity determined in the initial phase of cycle, which was a fatal problem to be solved. However, such problems are still left that storage property and reliability when left at a high temperature are not sufficiently achieved, and the specific capacity (mAh/g) and capacity density (mAh/cc) of the negative electrode are not satisfactory.
It is thus an object of the invention to improve further the reliability and high energy density of a lithium secondary cell.
To solve the aforesaid problems of the lithium ion secondary cell, according to the present invention, a carbonaceous powder of plural-layer structure with a surface layer of carbonaceous matter formed therein is used as a negative electrode material, said carbonaceous powder being prepared such that, using alumpy graphite powder as a nucleus, this nucleus or the graphite powder is covered with a carbon precursor, which is then fired in an inert gas atmosphere at a temperature within the range of 700 to 2800xc2x0 C., thereby causing the surface layer of the carbonaceous matter to form, wherein said lumpy graphite powder has the following characteristics:
(1) the plane interval (d002) of (002) plane by wide angle X-ray diffraction method is less than 3.37 angstroms, and the size (Lc) of crystallite in a C-axis direction is at least 1000 angstroms or more,
(2) R value, that is the peak intensity ratio of 1360 cmxe2x88x921 in relation to the peak intensity of 1580 cmxe2x88x921 in the argon ion laser Raman spectrum, is 0.3 or less, and the half width of 1580 cmxe2x88x921 peak is 24 cmxe2x88x921 or less,
(3) the mean particle size is 10 to 30 microns, and the thickness of the thinnest portion is at least 3 microns or more and not exceeding the mean particle size,
(4) the specific surface area by BET method is 3.5 m2/g or more and not exceeding 10.0 m2/g,
(5) the tapping density is 0.5 g/cc or more and not exceeding 1.0 g/cc, and
(6) the X-ray diffraction peak intensity ratio of (110)/(004) by wide angle X-ray diffraction method is 0.015 or more; whereby a nonaqueous electrolyte secondary cell of high specific capacity is realized, wherein the irreversible capacity noted in the initial cycle is made as small as possible, the storage property and reliability of the battery when left at a high temperature are enhanced, and excellent high rate discharge characteristic and discharge characteristic at a low temperature are achieved.